previous | next MRI of the brain, T1-weighted sagittal cut. 1, Posterior commissure. 2, Cerebral aqueduct (of Sylvius). 3, Tectum. 4, Fourth ventricle. 5, Cerebellum. 6, Quadrigeminal cistern. 7, Pineal gland. 8, Splenium, corpus callosum. 9, Third ventricle.